Reactive sputtering apparatus and cathode elements therefor

ABSTRACT

A reactive sputtering apparatus is disclosed having metal cathode units, each of which comprises a cathode element and an electrostatic shield spaced from and surrounding it on all sides except that which is to be presented towards a substrate on to which a coating of a compound of the cathode metal is to be sputtered, the electrostatic shield being formed with spaced inner and outer walls between which the sputtering atmosphere can be fed into the working space between the cathode element and the substrate. The functions of electrostatic shielding and gas supply are thereby separated from one another, so that the spacings of the inner wall from the cathode element, and of the inner wall from the outer wall, can be dimensioned to suit the different functions, and the risk of electrical breakdown can be much reduced.

United States Patent 11 1 Burrows et a1.

1 1 REACTIVE SPUTTERING APPARATUS AND CATHODE ELEMENTS THEREFOR [75] inventors: Kenneth Burrows; Robert Hiscutt,

both of Birmingham, England [73] Assignee: Triplex Safety Glass Company Limited, London England 1221 Filed: Apr. 11, 1974 211 App1tNo.:459,988

Grantham et a1 204/298 1 1 June 17, 1975 Primary Examiner-Oscar R. Vertiz Assistant E.\'uminerWayne A. Langel Attorney, Agent, or Firm-Sughrue, Rothwell, Mion. Zinn & Macpeak [57] ABSTRACT A reactive sputtering apparatus is disclosed having metal cathode units, each of which comprises a cathode element and an electrostatic shield spaced from and surrounding it on all sides except that which is to be presented towards a substrate on to which a coating of a compound of the cathode metal is to be sputtered. the electrostatic shield being formed with spaced inner and outer walls between which the sputtering atmosphere can be fed into the working space between the cathode element and the substrate. The functions of electrostatic shielding and gas supply are thereby separated from one another. so that the spacings of the inner wall from the cathode element, and of the inner wall from the outer wall. can be dimen sioned to suit the different functions. and the risk of electrical breakdown can be much reduced.

6 Claims, 6 Drawing Figures PATENTEDJUN 17 ms SHEET 5 W05 -Q ziomk mkm CATHODE T0 SHIELD GAP mm REACTIVE SPUTTERING APPARATUS AND CATHODE ELEMENTS THEREFOR CROSS-REFERENCE TO RELATED APPLICATION This invention relates to a feature which is particularly. though not exclusively, applicable to the apparatus disclosed in our co-pending U.S. application Ser. No. 220,889 dated Jan. 26. i972.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to electrostatically shielded metal cathode units for use in reactive sputtering apparatus. and to apparatus incorporating such cathode units.

2. Description of the Prior Art Reactive sputtering apparatus customarily comprises a vacuum chamber. means for supporting in the chamber a substrate on to whose surface a film of a metallic compound is to be sputtered. a cathode assembly arranged in the vacuum chamber in the vicinity of the substrate so as to present towards the substrate a surface of the metal whose compound is to be sputtered, means for applying a high negative potential to the cathode. and means for supplying a sputtering atmosphere of a reactive gas and another gas or gases (e.g. argon) at reduced pressure into the vacuum chamber. A glow discharge is effected between the cathode and substrate. as a result of which ions of the gas or gases forming the sputtering atmosphere (e.g. argon ions) bombard the surface of the cathode, thereby reactively sputtering a film of a compound of the metal and the reactive gas on to the surface of the substrate.

In the specification of our co-pending US Pat. application Serv No. 220.899 filed Jan. 26. i972, we have described and claimed a reactive sputtering apparatus for depositing a transparent. electrically conducting. metal oxide film on to the surface of a substrate of extended lateral dimensions. comprising a. a vacuum chamber b. means for supporting the substrate in the vacuum chamber.

c. means for maintaining the substrate at a controlled elevated temperature in the vacuum chamber.

d. means for supplying a sputtering atmosphere of oxygen and another gas or gases at a controlled reduced pressure into the vacuum chamber. e. a cathode assembly whose overall lateral dimensions are not substantially less than those of the substrate. arranged in the vacuum chamber in the vicinity of the substrate and presenting a plurality of spaced parallel strips comprising the metal whose oxide is to be deposited. extending parallel to the substrate surface. with passages between the strips for allowing the sputtering atmosphere to penetrate into the whole of the working space between the cathode assembly and the substrate.

f. means for applying a high negative potential of the order of l to K\' to the cathode assembly so as to cause deposition of the metal oxide film on the sub strate by reactive sputtering substantially perpendicularly from said parallel strips. and

g. means for causing relative translational movement between the cathode assembly and the substrate in a direction parallel to the substrate surface and transverse to the length ofthe parallel strips. through an amplitude substantially smaller than the overall length of the cathode assembly. but sufficient to cause all parts of the substrate to be coated by sputtering from at least one of said parallel strips for equal deposition periods during the deposition process.

In the preferred case, the parallel strips which form the individual elements of the cathode assembly are each provided with a respective electrostatic shield spaced therefrom. the sputtering atmosphere being fed into the working space through the spaces between the cathode strips and their respective electrostatic shields.

With such an arrangement, difficulty has been experienced in achieving adequate resistance to electrical breakdown between the cathode strip or element and its shield. At the low pressures used in reactive sputtering (of the order of 5 X 10 mm. Hg) and with a spacing between the cathode strip and the shield within the range which is practicable in such apparatus (say 2 to 20 mm.). the breakdown voltage decreases with in creasing gas pressure and with increasing cathode/- shield spacing. It only rises again at values of pressure and spacing which are too high to be used in practice. It also decreases if the spacing is reduced below 2 mm, due to field effects.

In the case of no gas flow, if the spacing is reduced within the practicable range of 2 to 20 min. the breakdown voltage will be increased due to the scarcity of ionizable gas molecules in a thin layer of gas at low pressure. In practice. however. if the flow rate of the sputtering atmosphere through the space between the cathode strip and its shield is adequate to maintain the concentration of the reactive gas in the working space. the increase in resistance to gas flow caused by a reduction in the spacing within the above range results in a local increase in the gas pressure and consequently the breakdown voltage remains low.

SUMMARY OF THE INVENTION According to the present invention, there is provided a cathode unit for use in a reactive sputtering apparatus, comprising a metal cathode element and an electrostatic shield spaced from and surrounding the cathode element on all sides except that which is to be presented towards a substrate on to which a coating of a compound of the cathode metal is to be sputtered, wherein the electrostatic shield is formed with spaced inner and outer walls between which, in use, a sputtering atmosphere can be fed into the working space between the cathode element and the substrate.

In this way, the functions of electrostatic shielding and supply of the sputtering atmosphere are separated from one another and the spacings of the double (i.e. inner and outer) shield walls from the cathode element and from one another can be dimensioned to suit the different requirements of the different functions. Since there is no gas flow between the cathode element and the inner wall of the electrostatic shield. the cathode element and the inner wall can be spaced apart at a distance such that the breakdown voltage is substantially higher than the potential applied to the cathode element in use. The outer wall of the electrostatic shield can be spaced from the inner wall at a distance such that the interspace between the walls affords any de sired cross-sectional area for accommodating the flow of the sputtering atmosphere. without having to consider the effect on the breakdown voltage. The shield nevertheless remains compact in size and of low weight.

The precise spacings used will depend upon the potential which is to be applied to the cathode element. the pressure in the working space and the flow rate of the sputtering atmosphere. For example. with an applied potential of 2 to k\" and a vacuum chamber pressure of the order of 5 X mm. Hg. the spacing between the cathode element and the inner wall of the electrostatic shield may be between 2 and 6 mm for an applied potential of 5 kV. the upper limit of the spacing increasing to l() mm if the applied potential is reduced to 2k\/'. With a throughput, which is defined as the product of pressure and volumetric flow rate. of the sputtering atmosphere of 0.8 litres. Torr/second [c.g. l6 litres/second at 0.05 Torr). the spacing between the inner and outer walls may be between 3 and 5 mm. The spacing between the inner and outer walls will in general be chosen so as to lead to substantial uniformity of gas supply along the edges of the cathode element.

The cathode unit may of course be one ofa plurality of similar units. of elongated shape. which together make up a cathode assembly. With such an elongated cathode unit. having a similarly elongated cathode ele ment. the electrostatic shield may comprise an inner trough-shaped side and bottom wall spaced from and surrounding bottom and side surfaces of the cathode element. an outer trough-shaped side and bottom wall spaced from said inner wall. and single end walls spaced from the ends of the cathode element, which close off the ends of both trough-shaped walls. Baffles may be disposed within the interspace between the inner and outer walls for enhancing uniformity of flow rates of the sputtering atmosphere along the length of the cathode unit. A pair of inlet pipes for the sputtering atmosphere may be secured to the outer wall at positions on the centre line of the cathode unit but spaced from one another along the length of the cathode unit and opening into the interspace between the inner and outer walls. two baffles being disposed parallel to one another and to the length of the cathode unit. one on each side of the said centre line. to restrict passage of the sputtering atmosphere laterally from the inlet pipes. The baffles may each be in the form of a plate-like member secured at right angles to the adjacent part of the outer wall and having a height which varies along the length of the baffle. being greatest in regions nearest to the inlet pipes.

The imention also resides in an apparatus for depositing a transparent. electrically conducting film ofa metallic compound on to the surface of a substrate by reactive sputtering, comprising a vacuum chamber, means for supporting the substrate in the vacuum chamber. a cathode unit including a metal cathode element arranged in the vacuum chamber so as to present towards the substrate a surface substantially parallel to the substrate surface and an electrostatic shield which is spaced from and surrounds the cathode element on all sides except that of the surface presented towards the substrate. means for connecting the cathode ele ment to a source of high negative potential, and means for supplying a sputtering atmosphere of a reactive gas and another gas or gases at reduced pressure through the electrostatic shield into the working space between the cathode element and the substrate. wherein the electrostatic shield is formed with spaced inner and outer walls between which the sputtering atmosphere is fed into the working space. the inner wall of the shield being spaced from the cathode element at a dis tance such that the breakdown voltage is substantially higher than the potential applied to the cathode element. The apparatus may comprise a plurality of such cathode units of elongated shape. together forming a cathode assembly which can be transversed parallel to the substrate surface and perpendicular to the length of the cathode units during the sputtering operation.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a reactive sputtering apparatus.

FIG. 2 is a diagrammatic side elevation of the cathode assembly of the apparatus of FIG. 1 showing the flow of the sputtering atmosphere through the eletrostatic shields of the individual cathode units,

FIG. 3 is a cross-sectional view to a larger scale of one of the cathode units used in the apparatus of FIG.

FIG. 4 is a longitudinal sectional view of the cathode unit of FIG. 3,

FIG. 5 is a sectional plan view of the cathode unit of FIGS. 3 and 4, and

FIG. 6 is a diagrammatic representation of the relationship between breakdown voltage and cathode/ shield spacing for different gas flow conditions.

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 illustrates a reactive sputtering apparatus which comprises a cylindrical vacuum vessel with removable vacuum-tight end closures (not shown]. The cathode assembly 27 comprises a plurality of spaced, parallel cathode units each comprising an element or strip 271, having an upper surface of indium/tin alloy, and an earthed electrostatic shield 28. The cathode as sembly 27 can be oscillated back and forth in the direction perpendicular to the length of the elements or strips 271, as described below. Each electrostatic shield 28, which surrounds its cathode element 271 on all sides except the upper side, is double-Walled and comprises an inner trough-shaped side and bottom wall 281, closely spaced from the side and bottom surfaces of the cathode element 271, and an outer troughshaped side and bottom wall 282. End walls 284 are common to both the inner and outer walls 281, 282 and close off the ends of the troughs. As shown diagrammatically in FIG. 2, the interspace between the walls 281, 282 forms a passage for the inflow of the sputtering atmosphere from a pipe 33 and branch pipes 331 into the working space 32 between the cathode assembly 27 and substrate 31. The sputtering atmosphere is thus fed into the working space from outlets extending along both sides of each elongated cathode element or strip 271.

Each of the elements 271 is hollow, as shown in FIGS. 3 and 4, its interior being filled with cooling water which is supplied through a flexible pipe 52 which enters near one end of the element. The water leaves through a second flexible pipe 51 near the other end of the element 271. The pipes 51.. 52 connect the elements 271 in series. but the pipes extending between the adjacent elements have been omitted from FIGS. 1, 2, 4 and 5 for clarity. The high-tension lead 44 from source 45 is of the co-axial cable type. the outer conductor being earthed and connected to both inner and outer walls 281 and 282 of the electrostatic shield 28, as shown in FIG. 3. Similar cables 44 connect the elements 271 to one another.

As shown in FIGS. 4 and 5. a pair of branch pipes 331 supplying the sputtering atmosphere are secured to the outer wall 282 at positions on the centre line of the cathode unit but spaced from one another along the length of the cathode unit. and open into the interspace between the inner wall 281 and the outer wall 282 of the electrostatic shield 28. The spacing between the walls 281 and 282 may be between 3 and 5 mm to provide a sufficient degree of resistance to the flow of the sputtering atmosphere to cause the flow rate to be made substantially uniform along the length of the cathode element or strip 271. If further uniformity of the flow rate along the length ofelement 271 is desired. longitudinal baffles in the form of two plate-like members or strips 283 may be disposed in the interspace between the walls 281. 282. As shown. the baffles 282 may be secured at right angles to the outer wall 282 and parallel to one another and to the length of the cathode unit. one on each side of the centre line of the unit. their height being varied along their length so as to be greatest in the regions nearest to the inlet pipes 331. They are thus profiled so as to produce a greater resistance to lateral flow in the regions X near the pipes 331 than in the regions Y remote therefrom (FIG. 4).

For a rectangular cathode element 271 ofdimcnsions 7.5 X 60 cm. the total flow rate may for example be l6 litres/second at a pressure of 0.05 Torr. through the electrostatic shield 28. The spacing between the inner wall 281 and the cathode element or strip 271 may be 3 mm for use with a potential of 2 to 5 kV and a vac- 28. in which space the pressure will be of the order of r 5 X l mm. Hg. and there is no gas flow to cause local increases in pressure.

The relationship between the breakdown voltage and the spacing between the cathode element and its shield. for a given low gas pressure of the order of X 10 mm. Hg and for four different conditions of gas flow. is illustrated in PK]. 6.

The curve A illustrates the relationship when there is no gas flow. Curve B represents the relationship when the product of gas pressure in the working space 32 and the rate of gas flow through the space between the cathode element and the shield is 0.4 litre. Torrlsecond. e.g. a flow rate of 8 litres/second and a pressure of 0.05 Torr. Curve C represents the relationship when the product of pressure and gas flow is 0.8 litres. Torr/second (e.g. l6 litres/second and 0.05 Torr.) and Curve D shows the relationship with a product of 1.6 litres. Torr/second. These figures apply to the rectangular cathode element of dimensions 7.5 X 60 cm. mentioned above.

It will be seen that when gas is flowing through the space between the cathode element and the shield, the breakdou n voltage reaches a maximum at a spacing of around 4 to 7 mm. depending on the gas flow. The maximum \aluc ofthe breakdown voltage is reduced as the gas flow is increased and in the conditions represented by curve C. which are most generally used. it is only 3 k\'. which is within the preferred operating range.

When there is no gas flow. however. as in the cathode elements of the present invention. and as represented by curve A. the breakdown voltage is much higher. For a convenient spacing of 3 mm. it is kV. well above the normal operating level. For operating at an applied potential of 2 kV. it can be seen from curve A that a spacing of up to 10 mm could safely be used. When operating at 5 kV. it is safe to use a spacing of up to 6 mm. The minimum spacing of 2 mm is determined by the appearance of field effects, as mentioned above.

Reverting to FIG. 1. only three cathode units comprising elements or strips 271 are shown for clarity. In practice, the number of cathode units used will depend on the length of the substrate to be coated. being generally chosen so that an oscillation having an amplitude equal to the spacing between the centre lines of the elements or strips 271 will cause all parts of the substrate to be covered. The cathode units are mounted on pairs of rollers 41 at each of their ends, and these rollers run on horizontal guide rails 42 secured to opposite sides of the vessel 40. The cathode units are connected to one another by adjustable link rods 43 which maintain their spacing and parallel alignment with one another and ensure that all the units can move together along the guide rails in the direction perpendicular to their length. A flexible high-tension lead 44 connects the cathode elements or strips 271 to the negative terminal of a high-voltage source 45.

A pair of pulleys 46 is mounted on a transverse shaft 47 at each end of the vessel 40 and a pair of traction wires or cables 48 connected at each end to the electrostatic shields 28 ofthe end units are led over the pulleys 46 to form drive means. One of the shafts 47 passes through the wall of the vessel 40 and is connected via a variable-amplitude oscillatory motion device 49 to an electric motor 50.

Above the horizontal guide rails 42. a pair of horizontal support rails 53 (only one of which is shown) are secured to opposite sides of the vessel 40 to support the substrate 31 which is to be provided with a transparent conducting film. Above the position of the substrate 31, a radiant heater 54 is secured in the vessel 40. fed through low-tension leads 55 and busbars 56 from a low voltage power unit 57. The heater 54 extends above the whole area of the substrate 31.

A thermocouple 58 is placed on the upper surface of the substrate 31 and connected through leads 59 to a calibrated dial instrument 60 to indicate the tempera ture of the substrate.

A vacuum pump (not shown) is connected to the interior of the vessel 40 through an exhaust connection 61 and a gas supply 62 of the selected atmosphere is connected through a flow meter 63 and needle valve 64 to a flexible pipe 65, connected to the supply pipe 33. Supply of the sputtering atmosphere direct to the working space 32 from between the walls 28], 282 of the electrostatic shields 28 assists in maintaining uniformity of the oxygen concentration in the working space. This has been found most important for ensuring that the deposited film has uniform and consistent proper ties of transparency and electrical conductivity. as de scribed in our co-pending application referred to above.

In use, when the substrate 31 has been placed on the support rails 53 and the end closures have been sealed. the vessel 40 is evacuated through the exhaust connection 61 and the selected sputtering atmosphere is supplied through the inlet 65, while the substrate is heated to the desired temperature by the heater 54. The cathode assembly 27 comprising the strips 271 is oscillated back and forth along the guide rails 42 by the motor 50 and the high negative voltage is applied to the strips 271 by the source 45. The vessel 40 and rails 42, 53, as well as the electrostatic shields 28. are carthed. A film of indium/tin oxides is thus sputtered on to the lower surface of the substrate 31. The heating effect on the substrate of the plasma in the working space is such that the heataing current supply from the low voltage power unit has to be reduced to maintain the substrate temperature constant within i 10C. of the desired value. An automatic control circuit of known type (not shown) can be used for this purpose.

The amplitude of the oscillatory motion of the strips 271 is adjusted to equal the spacing between the centre lines of the strips. This spacing can be adjusted by means of the link rods 43. All parts of the substrate 31 are effectively covered for equal deposition times by the strips during one part or another of each oscillatory cycle.

A substantially uniform highly transparent film oflow specific resistivity can thus be deposited on the substrate. Variations in the specific resistivity can readily be kept within ltl7r of a mean value.

As explained above. the double-walled construction of the electrostatic shield 28 gives an excellent protec tion against electrical breakdown while enabling the cathode assembly to be kept relatively compact.

We claim:

1. Apparatus for depositing a transparent, electrically conducting film of a metallic compound on to the surface of a substrate by reactive sputtering, comprising a vacuum chamber, means for supporting the substrate in the vacuum chamber. a cathode unit including a metal cathode element arranged in the vacuum chamber so as to present towards the substrate a surface substantially parallel to the substrate surface and an electrostatic shield inside said chamber which is spaced from and surrounds the cathode element on all sides except that of the surface presented towards the substrate, means for connecting the cathode element to a source of high negative potential, and means for supplying a sputtering atmosphere ofa reactive gas and another gas or gases at reduced pressure through the electrostatic shield into the working space between the cathode element and the substrate. wherein the electrostatic shield is formed with spaced inner and outer walls between which. in use. the sputtering atmosphere is fed into the working space.

2. Apparatus according to claim 1. wherein the cathode unit is of elongated shape. and wherein baffles are disposed within the interspace between the inner and outer walls for enhancing uniformity of flow rates of the sputtering atmosphere along the length of the cathode unit.

3. Apparatus according to claim 2, wherein a pair of inlet pipes for the sputtering atmosphere are secured to the outer wall at positions on the centre line of the cathode unit but spaced from one another along the length of the cathode unit and opening into the interspace between the inner and outer walls, two baffles being disposed parallel to one another and to the length of the cathode unit, one on each side of the said centre line. to restrict passage of the sputtering atmosphere laterally from the inlet pipes.

4. Apparatus according to claim 3, wherein the baffles are each in the form ofa plate-like member secured at right angles to the adjacent part of the outer wall and having a height which varies along the length of the baffle. being greatest in regions nearest to the inlet pipes.

5. Apparatus according to claim 1, wherein the cathode element is of elongated form and the electrostatic shield comprises an inner trough-shaped side and bottom wall spaced from and surrounding bottom and side surfaces of the cathode element, an outer troughshaped side and bottom wall spaced from said inner wall, and single end walls spaced from the ends of the cathode element, which close off the ends of both trough-shaped walls 6. Apparatus according to claim 1, comprising a plurality of said cathode units of elongated shape, together forming a cathode assembly which can be traversed parallel to the substrate surface and perpendicular to the length of the cathode units during the sputtering operation. 

1. Apparatus for depositing a transparent, electrically conducting film of a metallic compound on to the surface of a substrate by reactive sputtering, comprising a vacuum chamber, means for supporting the substrate in the vacuum chamber, a cathode unit including a metal cathode element arranged in the vacuum chambEr so as to present towards the substrate a surface substantially parallel to the substrate surface and an electrostatic shield inside said chamber which is spaced from and surrounds the cathode element on all sides except that of the surface presented towards the substrate, means for connecting the cathode element to a source of high negative potential, and means for supplying a sputtering atmosphere of a reactive gas and another gas or gases at reduced pressure through the electrostatic shield into the working space between the cathode element and the substrate, wherein the electrostatic shield is formed with spaced inner and outer walls between which, in use, the sputtering atmosphere is fed into the working space.
 2. Apparatus according to claim 1, wherein the cathode unit is of elongated shape, and wherein baffles are disposed within the interspace between the inner and outer walls for enhancing uniformity of flow rates of the sputtering atmosphere along the length of the cathode unit.
 3. Apparatus according to claim 2, wherein a pair of inlet pipes for the sputtering atmosphere are secured to the outer wall at positions on the centre line of the cathode unit but spaced from one another along the length of the cathode unit and opening into the interspace between the inner and outer walls, two baffles being disposed parallel to one another and to the length of the cathode unit, one on each side of the said centre line, to restrict passage of the sputtering atmosphere laterally from the inlet pipes.
 4. Apparatus according to claim 3, wherein the baffles are each in the form of a plate-like member secured at right angles to the adjacent part of the outer wall and having a height which varies along the length of the baffle, being greatest in regions nearest to the inlet pipes.
 5. Apparatus according to claim 1, wherein the cathode element is of elongated form and the electrostatic shield comprises an inner trough-shaped side and bottom wall spaced from and surrounding bottom and side surfaces of the cathode element, an outer trough-shaped side and bottom wall spaced from said inner wall, and single end walls spaced from the ends of the cathode element, which close off the ends of both trough-shaped walls.
 6. Apparatus according to claim 1, comprising a plurality of said cathode units of elongated shape, together forming a cathode assembly which can be traversed parallel to the substrate surface and perpendicular to the length of the cathode units during the sputtering operation. 